1. Field of the Invention
The present invention relates to well service tools. More particularly, the present invention relates to well service tools used for cutting or disintegrating objects in a wellbore such as casing, cement, or other debris lodged in the wellbore.
2. Summary of the Prior Art
Cutting tools for use in an existing wellbore, such as descalers, underreamers, tubing cutters, casing cutters, section mills, and the like, are conventional and well-known in the well service arts. Such tools typically are rotated to selectively cut or mill through objects disposed in a wellbore, and frequently are run into the wellbore through reduced-diameter conduits such as production tubing. Typically, these tools employ knives or cutting arms that are pivotally mounted to a portion of the well service tool. The knives may be selectively expanded radially outwardly from the tool to engage the object to be cut, milled, or reamed. The ends of the knives, which contact the object, typically are dressed with an abrasive material such as that disclosed in U.S. Pat. No. 4,978,260, issued on Dec. 18, 1990, to Lynde et al.
Canadian Patent No. 1,282,687, issued on Apr. 9, 1991, to Hailey discloses a prior-art well service tool. This patent discloses a pair of knives pivotally mounted to the body of the tool by means of a pin. The knives are arranged on the pin in a scissor fashion wherein an hydraulic piston acts on the uppermost surfaces of the knives to expand them radially outwardly from the tool body, into engagement with the object to be cut.
Such a typical prior-art well service tool has number of disadvantages. Because the knives are maintained in an extended position by a hydraulically actuated piston, the force necessary to retract the knives is a function of the fluid pressure exerted on the piston. If the force on the extended knives by cutting action exceeds the force exerted by the piston, the knives tend to retract during cutting operation and cutting engagement is lost, or an undergage hole is bored. Also, the knives are pinned to a major structural component of the well service tool. Because of the torsional forces exerted on the knives, the mounting pins and tool body surrounding the pins become the primary point of wear and failure. If the pins or tool body surrounding the pins fails, a major structural component of the tool is damaged and requires expensive and time-consuming replacement or repair.
Additionally, such direct piston-actuated well service tools may extend the knives in an uneven fashion, wherein one knife is further extended from the tool body than is another knife. Such uneven extension tends to result in an uneven, undergage cut, and may cause vibration and chattering of the well service tool and the workstring on which it is run.
The present invention overcomes these disadvantages by providing a well service tool for cutting, boring, or milling an object in a wellbore that has cutting arms or knives that are pivotally mounted to an expendable and easily replaced component of the well service tool. The present invention also provides a well service tool having a positive mechanism for extending the knives radially outwardly from the well service tool body and positively maintaining the knives in the extended position.